This invention relates to underfill materials for flip-chip type semiconductor devices.
With the advance of electric equipment toward smaller size, lighter weight and higher performance, the semiconductor mounting technology has changed from the pin mating type to the surface mounting which now becomes the mainstream. One bare chip mounting technology is flip-chip (FC) mounting. The flip-chip mounting is a technique of providing an LSI chip on its circuit pattern-bearing surface with several to several thousands of electrodes, known as bumps, of about 10 to 100 microns high and joining the chip to electrodes on a substrate with a conductive paste or solder. Then the sealing material used for the protection of FC devices must penetrate into gaps of several tens of microns defined by bumps between the substrate and the LSI chip. Conventional liquid epoxy resin compositions used as the underfill material for flip-chip devices are generally composed of a liquid epoxy resin, a curing agent and an inorganic filler. Of these, the most predominant is a composition in which a large amount of inorganic filler is blended in order to provide a matching coefficient of linear expansion with those of semiconductor chips, substrates and bumps for increased reliability.
With respect to stress properties, the flip-chip underfill materials with high loading of filler give rise to no problem. However, they suffer from very low productivity since they have a high viscosity due to the high filler loading so that they may penetrate into the gap between chip and substrate at a very slow rate. There is a desire to overcome this problem.
It is also known that when an underfill material is loaded with a large amount of inorganic filler, the particle size distribution and surface state of the filler largely affect the viscosity of the final product. Therefore, in the past, the filler is adjusted to an optimum particle size distribution, for example, by subjecting spherical silica (as produced by flame fusion) to air classification or sieving for removing coarse particles and fines therefrom, or by combining fractions of spherical silica having different particle sizes. Since the yields of these procedures are very low, the cost of raw material is increased.
It is also a common practice to treat the fillers, typically silica with surface modifiers such as silane coupling agents for improving the affinity of the silica surface to epoxy resins and hence, the bond strength therebetween. However, there arise a problem that even a minor amount of volatiles can cause to create voids since an underfill material is heat cured in a very narrow gap.
An object of the invention is to provide a flip-chip type semiconductor device underfilling material which maintains a low enough viscosity to ensure interstitial infiltration even when filled with a large amount of inorganic filler and which cures into a void-free, reliable product.
The invention is generally directed to an epoxy resin composition for use as an underfill material for flip-chip type semiconductor devices. It has been found that when spherical silica obtained by heating and burning spherical particles of polyorganosilsesquioxane, especially having a maximum particle size of up to 50 xcexcm and a mean particle size of 0.5 to 10 xcexcm and bearing carbon on the surface, is used as the inorganic filler, the resulting epoxy resin composition is improved in interstitial infiltration even when filled with a large amount of the inorganic filler. This is because the spherical silica defined above has an improved affinity to epoxy resins. As a result, semiconductor devices encapsulated with the composition are improved in reliability.
Accordingly, the invention provides an underfill material for flip-chip type semiconductor devices, comprising
(A) 100 parts by weight of a liquid epoxy resin,
(B) 100 to 300 parts by weight of spherical silica obtained by heating and burning spherical particles of polyorganosilsesquioxane, and
(C) 0.01 to 10 parts by weight of a curing accelerator.